1. Field of the Invention
The invention relates to silane-modified polyvinyl acetals, to a process for their preparation, and to their use.
2. Background Art
The preparation of polyvinyl acetals obtained from the corresponding polyvinyl alcohols by polymer-analogous reaction with appropriate aldehydes, was disclosed as early as 1924. Since then, aldehydes in wide variety have been used for preparing the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a 3-stage process in which polyvinyl acetate is first prepared and then hydrolyzed to the corresponding polyvinyl alcohol, which is then acetalized to form the polyvinyl acetal. The resultant products contain residual vinyl alcohol and vinyl acetate units as well as the vinyl acetal groups. Polyvinyl formal, polyvinyl acetacetal, and polyvinyl butyral (PVB) have achieved particular commercial importance. The term xe2x80x9cmodified polyvinyl acetalsxe2x80x9d hereinafter means polyvinyl acetals which contain other monomer-derived units in addition to vinyl acetate, vinyl alcohol, and vinyl acetal units.
The largest application sector for polyvinyl acetals is the production of safety glass in automotive construction and for architectural uses, plasticized polyvinyl butyral films being used as an intermediate layer in glazing units. Blends with modified polyvinyl butyral have also been proposed for this purpose, for example those having the acetal units described in EP-A 368832, bearing sulfonate, carboxylate, or phosphate functionality, which are said to improve blocking and flow performance. EP-A 634447 discloses modified polyvinyl butyrals whose main polymer chain contains monomer units which have sulfonate groups, these polyvinyl butyrals being obtainable via acetalization of polyvinyl alcohols having sulfonate functionality.
In EP-A 461399, polyvinyl butyrals modified with amino groups are disclosed. The latter polymers are used as precipitants. Another application sector for polyvinyl butyrals is their use in anti-corrosion coatings, as disclosed in EP-A 1055686, for example, where of polyvinyl acetals modified with tertiary alkanolamines are used for this purpose.
High pigment binding power makes polyvinyl butyrals eminently suitable as binders in paints, and particularly in printing inks. In printing inks, a further requirement is that organic solutions of the polyvinyl butyrals should have very low solution viscosity, in order to permit their use for producing inks with high solids content while using a very high binder content. Examples of polyvinyl butyrals having low solution viscosity include the modified polyvinyl butyrals disclosed in DE-A 19641064, obtained by acetalizing a copolymer having vinyl alcohol units and 1-alkylvinyl alcohol units.
A disadvantage possessed by all of the polyvinyl acetals described in the prior art is their inadequate adhesion to specific substrates. For this reason, the addition of coupling agents is essential in many cases. EP-B 0346768 describes the coating of films or foils with amino functional silanes such as N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, in order to improve bonding to other polymers, in particular to polyvinyl butyrals. In the case of polyethylene and polyester films, vinyltrimethoxysilanes or chloropropyltrimethoxysilanes have been used as adhesion promoters, providing successful lamination using hot-melt adhesives based on ethylene-vinyl acetate copolymers (E. Plueddemann, BONDING THROUGH COUPLING AGENTS, Plenum Press, New York, 1985). Primer coats of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane have been employed for improving the adhesion of an ionic resin, i.e. a salt of a polymer based on ethylene-methacrylic acid, to glass or polycarbonate films, as disclosed in U.S. Pat. No. 4,663,228. EP-B 0636471 claims a process for producing a glass composite, where adhesion between glass and polymer film, i.e. polyvinyl butyral, is improved by using a mixture of two or more silanes. One of the silanes is employed to increase the bond strength between the glass and the polymer film, while the other silane adhesion promoter is not selected for providing bonding between glass and polymer film.
The processes described in the prior art for improving adhesion between polyvinyl acetals and critical substrates by adding known coupling agents have clear disadvantages. For example, the adhesion-promoting action is often not sufficiently long lasting, or the composite weakens over time. Other disadvantages include yellowing of aminosilanes and their unpleasant odor and skin-irritant action, a known problem when using amino-functional compounds. Another problem lies in the actual mode of addition of the coupling agents, which always requires an additional, time-consuming step in the mixing procedure. The selection and addition of suitable coupling agents furthermore requires great expertise. For example, when the adhesion promoter is added to a polyvinyl acetal in organic solvent, the result can be incompatibility and inhomogeneity, in some cases to such a degree that phase separation may occur. In addition, the reactive silanes subsequently added as coupling agent may become involved in side reactions such as hydrolysis or condensation reactions. Another disadvantage consists in the large amount of adhesion promoter which generally must be added in order to substantially improve adhesion of polyvinyl acetals on critical substrates. The process can therefore become very expensive, since the cost of silanes is relatively high.
Four Japanese publications have disclosed silane-modified polyvinyl acetals. JP-A 06-247760 and JP-A 06-248247 disclose polyvinyl acetals modified with silane-functional monomer units for use as binders for cementitious compositions or for inorganic fiber materials. These Si-modified polyvinyl acetals are obtained by copolymerizing vinyl acetate with vinylalkoxysilanes, hydrolyzing the vinyl ester-vinylsilane copolymers, and finally acetalizing with aldehyde. JP-A 10-237259 proposes a mixture of silane-modified polyvinyl acetal and unmodified polyvinyl acetal for coating materials capable of being printed using ink-jet printers. In the latter reference, vinyl ester-vinylsilane copolymers are first hydrolyzed, are then mixed with unmodified polyvinyl alcohol, and finally acetalized with aldehyde. A disadvantage with the procedures found in these publications is that the result is always an inhomogeneous polymer in which pure polyvinyl acetal chains are present alongside polyvinyl acetal chains modified with silanol groups, which leads to incompatibility. The enrichment of silanol groups in the silane-modified polyvinyl acetal chains also increases the extent of condensation, which is undesired, and thus causes gelling. Only strongly alkaline systems, e.g. cementitious compositions, are capable of breaking down this gel to some extent, but in neutral organic solvents the inhomogeneity and the gel continue to be present, visible as marked phase separation, which is disadvantageous.
Finally, JP-A 62-181305 discloses a polyvinyl butyral modified with triethoxyisocyanatopropylsilane. Here, a polymer-analogous reaction is used to link the alkoxysilane group to the polymer skeleton via a urethane bond through reaction of the isocyanate group of the silane with free hydroxyl groups on the polyvinyl butyral. An associated disadvantage is that a reaction of this type is impossible in solvents containing hydroxyl groups, for example water and alcohols, since in such cases the isocyanate functions are hydrolyzed quantitatively with elimination of carbon dioxide to give the corresponding unreactive aminosilane.
An object of the present invention was to provide polyvinyl acetals which intrinsically have very good adhesion to various substrates and in particular to the known critical substrates, making it unnecessary to add any coupling agent at all. It has been surprisingly discovered that polyvinyl acetals obtained from polyvinyl alcohols via polymer-analogous reaction with aldehydes, where at least one aldehyde, optionally present in the form of its hemiacetal or acetal, contains hydrolyzable silane groups, have markedly improved adhesion to critical substrates, to the extent that it is indeed possible to omit the use of coupling agent. There is also a great improvement in adhesion to glass or metals. The adhesion-improving effect of these modified polyvinyl acetals begins to occur even at a very low content of alkoxysilane, alkoxysiloxane, and/or silanol groups.